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Creating Unconventional Metals

The semiconductor silicon and the ferromagnet iron are the basis for much of mankind’s technology, used in everything from computers to electric motors. In this week’s issue of the journal Nature (August 21st) an international group of scientists, including academic and industrial researchers from the London Centre for Nanotechnology (UK), USA and Lesotho, report that they have combined these elements with a small amount of another common metal, manganese, to create a new material which is neither a magnet nor an ordinary semiconductor. The paper goes on to show how a small magnetic field can be used to switch ordinary semiconducting behaviour (such as that seen in the electronic-grade silicon which is used to make transistors) back on.

The new material exists in a quantum halfway house between magnet and semiconductor - in the same way that much more complex materials such as ceramics which exhibit high temperature superconductivity exist in quantum halfway houses between metals and magnetic insulators. The research is of fundamental importance because it demonstrates, for the first time, a simple recipe for reaching this halfway house, whilst also suggesting new mechanisms for controlling electrical currents and magnetism in semiconductor devices.

Professor J.F. DiTusa of Louisiana State University and a coauthor of the paper said: “It’s amazing that something which was thought to exist theoretically in mathematical physics could actually be found in an alloy which was simply formed by melting together a few common elements.”

Professor Gabriel Aeppli of UCL (University College London), another member of the research team and Director of the London Centre for Nanotechnology, added: “It might be possible to see similar effects in devices made using materials and methods of the type used to make laser pointers. This would put what we’ve seen firmly in the realm of that which can easily be achieved using current technologies.”

The first author of the paper, Dr. N. Manyala of the National University of Lesotho, said: “We are looking forward to investigating whether we can see these effects using thin layers of the same materials deposited directly on the silicon wafers. These wafers are the same as those used by mass market electronics manufacturers as the basis for integrated circuits.” Dr. Ramirez, who is now with LGS-Bell Labs Innovations echoed this thought, noting that, "with the end of Moore's law in sight, mechanisms for controlling and understanding possible new information bits such as spins in solids are actively being sought after."

Fig. 1: An unconventional metal: The magnetic bar magnets (called “magnetic moments”) associated with the mobile electrons (red arrows) responsible for electrical conduction and manganese atoms (green arrows) in manganese doped iron silicide (Fe1-xMnxSi). This figure depicts the coupling of the magnetic moments as the temperature is reduced from room temperature (top of the figure) where the magnetic dipoles are independent, to very low temperature (bottom of the figure) where coupling between the dipoles creates regions where the moments add to zero (light blue region). The existence of a population of uncoupled complexes (depicted here in the yellow region) down to the lowest temperatures results in the material being neither a magnet nor common semiconductor. External magnetic fields align these rare yellow regions to the magnetic field, switching on ordinary semiconducting behaviour. (Image available from UCL Press Office, contact information below)

The London Centre for Nanotechnology is an interdisciplinary joint enterprise between University College London and Imperial College London. In bringing together world-class infrastructure and leading nanotechnology research activities, the Centre aims to attain the critical mass to compete with the best facilities abroad. Research programmes are aligned to three key areas, namely Planet Care, Healthcare and Information Technology and bridge together biomedical, physical and engineering sciences. Website: www.london-nano.com

About UCL (University College London):
Founded in 1826, UCL was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender, and the first to provide systematic teaching of law, architecture and medicine. In the government's most recent Research Assessment Exercise, 59 UCL departments achieved top ratings of 5* and 5, indicating research quality of international excellence. UCL is in the top ten world universities in the 2007 THES-QS World University Rankings, and the fourth-ranked UK university in the 2007 league table of the top 500 world universities produced by the Shanghai Jiao Tong University. UCL alumni include Marie Stopes, Jonathan Dimbleby, Lord Woolf, Alexander Graham Bell, and members of the band Coldplay. Website: www.ucl.ac.uk

About Louisiana State University:
LSU is the flagship institution of the state of Louisiana and is one of only 21 universities nationwide holding land-grant, sea-grant and space-grant status. Since 1860, LSU has served the people of Louisiana, the region, the nation, and the world through extensive, multipurpose programs encompassing instruction, research, and public service. The University brings in more than $120 million annually in outside research grants and contracts, a significant factor for the Louisiana economy. Website: www.lsu.edu

About The National University of Lesotho:
The National University of Lesotho is a growing institution striving to meet the needs of the nation, through producing competent and skilled graduates who can easily take up the call to assist in the development of Lesotho. The 80 hectare University site is situated at Roma (pop.8,000) some 34 kilometers south-east of Maseru, the capital of Lesotho. Roma valley is broad and is surrounded by a barrier of rugged mountains which provides magnificent scenery. The University enjoys a temperate climate with four distinctive seasons. Website: www.nul.ls

About LGS Bell Labs Innovations:
LGS is the successor to the former Lucent and Alcatel Government Solutions business units. Beginning operations on January 1, 2007, LGS is an independent and wholly-owned subsidiary of Alcatel-Lucent's North American operations. Alcatel-Lucent is the leading provider of telecommunications & networking products and services worldwide. Delivering the promise of ideas through the power of Bell Labs technology, LGS continues to play a prominent role, on behalf of the U.S. Government, in preserving the technological preeminence of America.

We have a long and proven heritage of delivering highly reliable and secure network technology, and breakthroughs in the way the Federal Government communicates amongst itself, with its constituency and other jurisdictions, and around the world. LGS also has proven experience in forming alliances and partnerships with leading defense contractors, system integrators, and service providers. Website: www.lgsinnovations.com